The present invention relates generally to composite structures and fabrication methods, more particularly to improved composite structures having prestressed fibers.
In a laminated composite material, temperature changes from fabrication to use mid differences in thermal expansion coefficients between the fiber and matrix materials combine to produce stresses in the composite layers which reduce the load which can be applied before onset of composite damage (first ply failure ). Because the temperature range over which composites are used is wide, stress levels produced as a result of processing may reduce the ultimate layer strength. Moreover, in some composites, such as carbon fiber composites, the coefficient of thermal expansion of the fiber along the fiber direction is different from that along a fiber radius so that a composite with all fibers oriented in the same direction will contract in the fiber direction differently from in the perpendicular direction. Composites typically comprise a plurality of unidirectional fiber layers, each layer having a preselected angular orientation with respect to adjacent layers to form a structure having desired stiffness and strength. Each layer may therefore contract differently from an adjacent layer with resulting stresses between layers.
In accordance with a governing principle of the invention, the three-dimensional stress state within a cured composite may be substantially relieved by prestressing the fibers during cure; prestressing also improves fiber linearity hi the composite. In composite fabrication according to the invention, a matrix impregnated fiber tow is tightly wound with preselected tension on an adjustable mandrel. The composite is conventionally cured at appropriate temperature in an autoclave with the fibers under tension. The degree of contraction of the fibers and matrix after cure depends on the magnitude of the tension applied to the fibers. The tension is induced by a combination of mechanical and thermal expansion of the mandrel. Stress fields with the composite are thereby modified, as compared to a composite conventionally processed, to a degree dependent on the magnitude of tension applied to the fibers.
It is therefore a principal object of the invention to provide an improved composite fabrication method.
It is a further object of the invention to provide a composite fabrication method comprising prestress of the composite fibers.
It is another object of the invention to provide a composite structure having improved strength and fiber linearity.
These and other objects of the invention will become apparent as a detailed description of representative embodiments proceeds.